Automotive instrument panels and door panels are typically composites which are made of a rigid backing which supports a semi-rigid urethane foam with the semi-rigid urethane foam being covered with a skin compound. Such skin compounds are typically blends of polyvinyl chloride (PVC) with a nitrile rubber (NBR). The nitrile rubber is included in such blends as a permanent modifier for the PVC which provides it with a higher degree of flexibility. The automotive industry is currently moving toward more aerodynamic body designs which typically include larger glass areas. Such design changes have significantly increased the heat and ultraviolet light aging requirements of automotive interiors. This has in turn significantly increased the demands put upon the polymers which are utilized as skins in automotive interior panels.
Heat and light stabilizers can be employed to improve the heat and ultraviolet light aging characteristics of conventional PVC/NBR blends which are utilized as skins for automotive interior panels. However, the degree to which the aging characteristics of such blends can be improved by the addition of additives is limited. In fact, there is a demand for performance characteristics in such applications which heretofore has not been realized by the utilization of heat and light stabilizers.
It is highly desirable for the skins used in automotive panels to resist discoloration and cracking under conditions of high heat and intense ultraviolet light throughout the life of the vehicle. Low fogging behavior is also of great importance to the automotive industry. Fogging is caused by the condensation of volatile compounds which vaporize from the interior panel onto the windscreen or side windows of the vehicle.
NBR/PVC blends offer an array of properties which make them useful as a skin composition for automotive panels. The NBR acts as a permanent flexibilizing monomer for the PVC. It also acts as a shrinkage control agent, and embossing aid, and improves grain retention. The NBR in such blends further provides vacuum-forming gauge control and exhibits low fog characteristics. NBR is highly compatible with PVC and has the capability of being recycled. It is essential for any polymer which is substituted for NBR to display these essential characteristics.
Rubbery polymers that can be blended with PVC to make leathery compositions having good heat and ultraviolet light resistance are described in U.S. Pat. Nos. 5,674,933, 5,616,651 and 5,415,940. These rubbery polymers are comprised of repeat units which are derived from (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) acrylonitrile, (d) styrene, (e) a half ester maleate soap or a dispersant selected from the group consisting of aromatic formaldehyde condensation products and polycarboxylates and (f) a crosslinking agent. Rubbery polymers of this type are sold by The Goodyear Tire & Rubber Company as Sunigum.RTM. rubber. However, rubbery polymers of this type often possess an undesirable odor caused by the presence of residual monomers.
Numerous technique can be used to reduce the level of residual monomers in such rubbery polymers. For instance, U.S. Pat. No. 5,674,933 describes a technique for deodorizing such rubbery polymers by adding an aminoalcohol to the latex prior to coagulation. One drawback to this approach is that it introduces the aminoalcohol into the rubbery polymer and adds cost.